Various radiation sensing and infrared imaging devices and systems are known in the art. Such systems can be constructed using arrays of MIS devices for sensing incident radiation. See, for example, U.S. Pat. Nos. 3,805,062 and 3,983,395 assigned to the assignee of the present invention and U.S. Pat. No. 4,327,291. In systems based upon CIDs, it is necessary to isolate each detection center in the array from the other detection centers of the array. Typically each detection center is isolated from its nearest neighbors by a field plate which surrounds the detection center and provides a channel stop. This field plate is normally biased so that the substrate surface beneath it is in accumulation. In this context, an edge field enhancement effect which will be discussed below has been seen to be a limiting factor in extending the dynamic range of many infrared imaging systems.
It is well known that the field produced at an edge or a corner of an electrode is much higher than that produced at a planar surface portion of the same electrode which is remote from such edges or corners. In an MIS structure, the increased edge field at the edge of a biased electrode or gate penetrates through the insulating layer into the semiconductor substrate resulting in an edge field enhancement effect at the edge of the potential well formed in the portion of the substrate beneath the MIS electrode. This edge field enhancement occuring in the portion of the substrate underlying the edge of the electrode is exacerbated when a field plate surrounding the electrode is biased so that the substrate surface beneath the field plate is in accumulation. The edge field enhancement tends to produce a sharply increased leakage current at the well edge which likely occurs as a result of band-to-band tunneling. The increased leakage current partially fills the potential well with unacceptable noise. To prevent this introduction of noise, the amplitude of the read out or injection voltage pulses applied to the electrode of an MIS device which is part of an imaging system must be significantly reduced from that amplitude which theoretically could be applied in the absence of the edge field enhancement effect. This limitation on the injection voltage limits the well capacity and the maximum charge which can be read out of the MIS device.
In order to improve the dynamic range of many presently known imaging systems employing MIS devices, it is desirable to substantially eliminate any leakage current resulting from the edge field enhancement effect so that an increased injection voltage can be applied without an unacceptable increase in noise charge. Since infrared sensors must be able to function over a wide dynamic range and resolve signals including a high level of background noise, attaining an improved dynamic range and a high well capacity is particularly important for such sensors. Further, infrared sensing devices are often constructed using narrow band gap semiconductor materials, such as indium antimonide (InSb) or mercury cadmium telluride (HgCdTe), which intrinsically have a lower breakdown voltage than wider band gap materials such as silicon (Si). Thus, the problem of achieving a reduced edge field enhancement and a high well capacity is compounded for many infrared detecting systems and is a limiting factor in the further development of many such systems.
Various techniques have been developed in attempts to reduce the edge field enhancement effect. U.S. Pat. No. 4,327,291 applies the well known technique for minimizing electric field problems at sharp corners by eliminating the sharp corners. U.S. Pat. No. 4,327,291 also seeks to minimize electric field problems by adjusting the positioning of the field plate and gate electrodes. A further attempt to reduce edge field enhancement involves adjusting the contour of a gate electrode so that it is nonplanar. While these various geometrical adjustments may result in a small increase in well capacity and a slight edge field decrease, such adjustments may add complexity and cost to the fabrication process or even degrade other pertinent performance characteristics such as lag or crosstalk.
A further technique for reducing the edge field enhancement problem is to construct an MIS device, such as an MIS capacitor, with a field plate which surrounds the MIS electrode and to bias the field plate up to the flatband voltage, the voltage at which depletion is about to begin in the semiconductor surface beneath the field plate. By so biasing the field plate, the bias voltage which can be applied to the MIS electrode can be increased somewhat. However, additional increase cannot be achieved by further increasing the bias on the field plate without defeating the purpose of the field plate because once the flatband voltage is exceeded, depletion or inversion begins and the level of isolation provided by the field plate rapidly decreases and becomes nominal as surface conduction begins. Consequently, the technique of biasing the field plate may only be used to achieve a small reduction in the edge field enhancement problem.
Accordingly, it is an object of this invention to provide method and apparatus suitable for achieving infrared imaging systems with improved dynamic range.
Another object is to provide MIS and CID devices having edge field enhancement which is significantly reduced from that observed with devices as described above.
Another object is to provide MIS and CID devices having an increased maximum injection voltage and having an increased maximum charge which can be read out therefrom.
Another object is to achieve the above objects with a device which can be simply and inexpensively fabricated using planar fabrication techniques.
These and other objects will be apparent from a consideration of the following detailed description and the accompanying claims.